This invention relates generally to correcting pointing errors in satellite data and particularly to correction of errors caused by a satellite's momentum wheel oscillations which are coupled into the satellite spin rate.
Many earth-orbiting satellites, such as surveillance and communication satellites, must be oriented so as to point to the earth. In general, the z-axis of the satellite is maintained so as to line up with the center of the earth. In addition, a surveillance satellite rotates about its z-axis so that a sensor array can detect targets, usually those on or near the earth's surface.
A satellite, however, tends to maintain its orientation in space and rotational forces are required to change its direction to keep it pointing toward the earth. Such rotational forces can include positioning impulse jets which are pulsed occasionally so as to keep the satellite's z-axis pointing to the earth. There are also reaction or momentum wheel and magnetic dipole generators for changing the satellites attitude with respect to the earth or other body about which they orbit.
The action of the impulse jets (or other forces) cause a change in the angular momentum of the satellite and a time rate of change of angular momentum creates a torque, i.e., a force moment that acts on the satellite, causing it to rotate about a third axis. The third axis rotation, called precession, is a result of the angular momentum of the satellite spin and the torque resulting from the force of the positioning jets. For example, with the satellite spinning about its z-axis and the positioning jets rotating the satellite about its y-axis, the precession torque causes the satellite also to rotate about its x-axis. Precession acts to prevent a spinning body from being moved relative to only one of its axes. The motion is, in other words, the positioning rotation is translated by 90.degree. in the direction of the satellite rotation. Vectorially, precession is the cross-product of the satellite's spin and the correcting force.
To reduce or to eliminate precession, the angular momentum of the satellite rotating about its z-axis is cancelled by an on-board momentum wheel having its axis of rotation coincident with the satellite's z-axis but rotating in the opposite direction. The momentum wheel's angular velocity is ideally constant as is the satellite spin rate. External forces, however, tend to slow the satellite spin which is restored, for example, by jet thrusters.
Coulomb forces, principally random frictional drag in the bearings, causes changes in the angular velocity of the momentum wheel. The changes are detected by a tachometer coupled to the momentum wheel's shaft. A control system uses the tachometer input signal to regulate the momentum wheel velocity.
The result is spurious oscillations being superimposed on the satellite spin rate. These disturbances must be corrected to preclude satellite pointing errors. The corrections are the subject of the present invention.
Prior art references are directed to attitude control and compensation. For example, U.S. Pat. No. 5,020,745 is directed to reducing attitude errors in a spacecraft using reaction wheels when the reaction wheel's direction of rotation is reversed. (Reaction wheels in the literature refer to those rotating masses which provide control by rotating in either direction whereas momentum wheels are those rotating masses which rotate in the same direction, providing control in some instances by varying the speed of rotation.) Attitude errors are introduced as the reaction wheel comes to rest and the direction of rotation is reversed.
U.S. Pat. No. 3,937,423 is directed to three-axes attitude control using one rotatable momentum wheel in one degree of freedom and jet thrustors in the degrees of freedom.
U.S. Pat. No. 5,058,835 is a system using four reactor wheels for attitude control and develops integrated vector errors to added to the attitude control torque commands.
U.S. Pat. No. 5,428,118 describes a momentum wheel having a rotational speed kept above a threshold value to insure proper lubrication. The problems of proper lubrication of a momentum wheel's bearings is explained.
U.S. Pat. No. 5,308,024 describes attitude control with disturbance torque compensation for stabilizing three-axis, zero-momentum satellites such as communication satellites which normally do not rotate but are earth-facing.
U.S. Pat. No. 4,567,564 is directed to attitude in stabilization in flexible space vehicles. It utilizes an observer (Kalman Filter) and a state controller to correct for weakly-damped bending vibrations of the vehicle.
U.S. Pat. No. 5,269,483 uses two loops--a fast loop and a slow loop--for attitude control.
U.S. Pat. No. 4,521,855 also uses a fast control loop and a slow control loop for continuous yaw correction with three momentum wheels.
U.S. Pat. No. 4,786,018 uses attitude variation signals applied to a Kalman Filter which supplies actuator control signals with feedback to a modeling circuit.
U.S. Pat. No. 5,259,577 uses a polar star sensor and a momentum wheel along the pitch axis with magnetic dipole generators in the roll and yaw planes.
While these prior art references are suited to the purpose for which they are intended and provide some valuable background about the problems of satellite attitude correction and control, they address the correction of pointing errors only in an indirect manner. They do not suggest the present approach to correcting the more subtle pointing errors introduced by the effect of Coulomb forces on the speed of a momentum wheel.
In accordance with the invention, a spacecraft having momentum wheel includes follower means to indicate the motion of the momentum wheel. An observer responds to signals from the spacecraft to supply calculated yaw rate and yaw values of the spacecraft. A detector responsive to the follower means supplies values of variations in the rate of rotation of the momentum wheel and the values of the variations are applied from the detector means to the observer means to correct the calculated spacecraft yaw rate.
The utility of the invention is corrections of errors in the determined spin rate of a space vehicle and resulting pointing errors caused by undesirable perturbations in the angular velocity of an on-board momentum wheel.